1. Field of the Invention
The present invention relates to a serial data transmission device which can find instantaneously an abnormal state such as a line breakage in a transmission line, and a terminal unit suitably used for the same.
2. Description of the Related Art
In various industrial machines and robots, it is necessary that many sensors and actuators are connected to a controller to perform data transmission between them. In order to realize a data transmission between a controller acting as a master unit, and a sensor and an actuator each acting as a terminal unit, a serial transmission system which can reduce the number of cables has been conventionally employed. For such a serial data transmission system, the conventional configuration shown in FIG. 12 has been developed to detect an abnormal transmission line such as line disconnection.
That is, the signal line L derived from the master unit M is connected to the first terminal unit (sensor or actuator) T1. The terminal unit T1 is connected to the second terminal unit T2 via the crossover line L1. The terminating unit Te is connected to the last terminal unit Tn by using repeatedly many crossover lines L1, L2, . . . , Ln.
An address is allocated to each of the terminal units. The master unit M outputs sequentially and serially address information and data to interchange between the terminal units. In this case, the master unit M varies sequentially a specified address to circulate the terminal units T1, T2, . . . , Tn. Data exchange are repeated between the master unit M and the terminal units T1, T2, . . . , Tn by repeating the circulation several times at a predetermined period.
The terminating unit Te has a function that monitors clock pulses for an address specification to detect that a transmission per cycle has completed. When the completion is detected, a confirmation signal is output to the signal line L via the crossover lines Ln, . . . , L2, L1. If there is an abnormal state such as line breakage in the crossover lines Ln, . . . , L2, L1 or the signal line L, the confirmation signal does not return to the master unit M. Hence the master unit M judges that a line breakage has happened to execute an error shooting process such as an alarm output, a system halt.
However, the above structure performs a line disconnection detection by judging merely whether the confirmation signal has returned. Hence even if the master unit M should detect the fact that there is a line breakage in the signal line or the crossover line, it cannot locate where the line disconnection spot of the transmission line is. In order to locate the abnormal occurrence spot, the whole transmission line must be inspected. For that reason, there is a problem that a quick recovery work is disturbed.
A line disconnection detection system which can instantaneously locate a disconnected line spot, for example, is disclosed in Japanese Laid-open Patent Publication No. 62-173830. In this prior art configuration, since the master unit varies sequentially a specified address to circulate respective terminal units, thus communicating sequentially to each of terminal units. The circulation is made several times at a predetermined period to repeat the data exchange between the master unit and each terminal unit.
In this prior art configuration, a confirmation signal is returned from each terminal unit to the master unit every time a data transmission to each terminal unit has been completed. Therefore, in the case where there is a line disconnection in a spot in a transmission line, a confirmation signal is not returned from the terminal unit connected to the transmission line to the master unit so that the line disconnection occurrence position can be easily located.
However, in the prior art disclosed in the Japanese Patent Publication, since a confirmation signal is returned to the master unit every time a data transmission is made to each terminal unit, the master unit has to wait for the transmission to the terminal unlit corresponding to the next address to confirm the confirmation signal returned from each terminal unit. As a result, there is a disadvantage in that the transmission efficiency per unit time is decreased.